Tropical Depression TWELVE (Text)

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TROPICAL DEPRESSION TWELVE DISCUSSION NUMBER 1
NWS NATIONAL HURRICANE CENTER MIAMI FL AL122011
500 AM AST MON AUG 29 2011
THE AREA OF LOW PRESSURE IN THE FAR EASTERN ATLANTIC TO THE SOUTH OF
THE CAPE VERDE ISLANDS HAS GAINED SUFFICIENT CONVECTIVE
ORGANIZATION TO BE CLASSIFIED AS A TROPICAL DEPRESSION. A CURVED
CONVECTIVE BAND WRAPS AROUND THE WESTERN AND SOUTHERN SIDE OF THE
SYSTEM AND THE INITIAL INTENSITY IS SET TO 30 KT BASED ON THE 06Z
DVORAK CLASSIFICATION FROM TAFB. THE DEPRESSION IS CURRENTLY IN AN
EASTERLY SHEAR ENVIRONMENT SITUATED TO THE SOUTH OF AN UPPER-LEVEL
RIDGE OVER THE EASTERN ATLANTIC. SOME EASTERLY SHEAR IS EXPECTED TO
PERSIST...BUT OTHERWISE THE ENVIRONMENT SHOULD BE CONDUCIVE FOR
STRENGTHENING THROUGH THE PERIOD. THE OFFICIAL INTENSITY FORECAST
SHOWS SLOW STRENGTHENING IN GOOD AGREEMENT WITH THE SHIPS...GFDL...
AND HWRF MODELS THROUGH 48 HOURS. AFTER THAT TIME THE CYCLONE SHOULD
STRENGTHEN A LITTLE MORE QUICKLY AS IT MOVES OVER PROGRESSIVELY
HIGHER OCEAN HEAT CONTENT. AT DAYS 3 THROUGH 5 THE NHC FORECAST IS
CLOSEST TO BUT A LITTLE BELOW THE HWRF MODEL. THE SHIPS FORECAST
LATE IN THE PERIOD APPEARS TOO LOW DUE TO STRONG SOUTHERLY SHEAR
IMPARTED BY THE TROPICAL CYCLONE VORTEX IN THE GFS MODEL FIELDS...
WHICH IS DISPLACED TO THE SOUTH AND EAST OF THE OFFICIAL TRACK.
THE INITIAL MOTION ESTIMATE IS TOWARD THE WEST OR 270 DEGREES AT 13
KNOTS. THE DEPRESSION IS CURRENTLY SITUATED TO THE SOUTH OF THE
SUBTROPICAL RIDGE...BUT THE WESTERN EDGE OF THE RIDGE WILL WEAKEN
OVER THE NEXT DAY OR TWO AS A MID/UPPER-LEVEL LOW CUTS OFF AND
RETROGRADES ACROSS THE CENTRAL ATLANTIC. THIS WILL ALLOW THE
DEPRESSION TO TURN TOWARD THE WEST-NORTHWEST DURING THE NEXT DAY OR
SO. AFTER 48 HOURS...THE MID/UPPER-LEVEL-LOW WEAKENS AND THE RIDGE
RE-STRENGTHENS TO THE NORTHEAST OF THE CYCLONE...WHICH SHOULD
RESULT IN A CONTINUED WEST-NORTHWESTWARD MOTION THROUGH THE END OF
THE PERIOD. THE OFFICIAL NHC TRACK FORECAST IS SOUTH OF AND FASTER
THAN MOST OF THE GUIDANCE FOR THE FIRST 48 HOURS...AS MOST OF THE
DYNAMICAL MODELS SHOW A MORE POLEWARD AND SLOWER MOTION THAN IS
CURRENTLY OBSERVED. DURING THIS TIME THE NHC TRACK IS BETWEEN THE
DYNAMICAL MODEL CONSENSUS AND THE LBAR...WHICH TYPICALLY PERFORMS
WELL IN THE DEEP TROPICS. AT 72 HOURS AND BEYOND THE NHC FORECAST
IS CLOSE TO A BLEND OF THE HWRF...GFDL...AND ECMWF MODELS AND IS
MUCH FASTER THAN THE LATEST RUN OF THE GFS.
FORECAST POSITIONS AND MAX WINDS
INIT 29/0900Z 9.4N 26.3W 30 KT 35 MPH
12H 29/1800Z 9.8N 28.3W 35 KT 40 MPH
24H 30/0600Z 10.7N 30.9W 40 KT 45 MPH
36H 30/1800Z 11.9N 33.5W 45 KT 50 MPH
48H 31/0600Z 13.1N 36.3W 50 KT 60 MPH
72H 01/0600Z 15.0N 42.5W 65 KT 75 MPH
96H 02/0600Z 16.5N 48.0W 75 KT 85 MPH
120H 03/0600Z 18.0N 52.0W 85 KT 100 MPH
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FORECASTER BRENNAN
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